A light-emitting device utilizing light emission from an electroluminescent element (light-emitting element) has been attracted attention as a display device or a lighting device.
As a light-emitting element used for a light-emitting device, a light-emitting element including a layer containing light-emitting compounds interposed between a pair of electrodes is well known.
Within such a light-emitting element, either of the electrodes serves as an anode, and another serves as a cathode, holes injected from the anode and electrons injected from the cathode are recombined with each other to form molecular exciton, and the molecular exciton radiates energy as light while returning to the ground state.
The demand for low power consumption is especially increased in display devices to be installed in various information processing devices, which have been drastically developed in recent years. In order to achieve low power consumption, it has been attempted to reduce a voltage for driving a light-emitting element. In consideration with commercialization, it is important not only to reduce a voltage for driving a light-emitting element but also extend a lifetime of a light-emitting device. Therefore, a light-emitting device has been developed to achieve low power consumption and a long lifetime.
For example, unexamined patent publication No. 9-63771 discloses that a voltage for driving a light-emitting element is reduced by forming an anode by a metal oxide having a large work function such as a molybdenum oxide. Moreover, a lifetime of the light-emitting element can be extended.
The means for extending a lifetime of a light-emitting element disclosed in the unexamined patent publication No. 9-63771 is insufficient, and so it is required to develop the technique for achieving a further long lifetime.